The present invention relates to a method of granulating water soluble fertilizers with high kieserite content.
The plant nutrient magnesium has been of increased importance in last years in agriculture and forestry, since many soils are substantially depleted of magnesium on different reasons. The naturally present magnesium sulfate mineral kieserite has been recognized as especially suitable for elimination of acute deficiency of such soils. In comparison to consumption time also for plants with short vegetation time, the kieserite provides for fast solution, despite the fact that its speed of solubility in laboratory measurements is lower than for other easily water soluble fertilizer salts. Since it permits to obtain higher ion concentration in the soil solution than other compositions of magnesium, the magnesium intake by the plants is substantially facilitated even in stress conditions.
The modern processing of natural crude salt produces a crystalline kieserite product with a grain size between 0.1 and 1 mm, which is approximately dust free and therefore, is favorable for environmental protection during application on fields and in forests. However, for application devices which are used now, this grain size is disadvantageous. It is, therefore, required to increase the size of the kieserite grains by granulation, so that it would be suitable for grained fertilizers and for dispersion characteristic of the application devices.
The granulation of pure kieserite and easily soluble fertilizers which include a kieserite in quantity more than one fourth of their mass is unexpectedly a very difficult operation, despite the fact that kieserite in combination with fertilizing materials unsoluble in pure water, such as for example slag and/or rock milling, is known as a preferable binder. This is disclosed in the German patent application P 3,541,184.8. This fact is associated with the physical and chemical properties of the kieserite and of the substances with which it is combined. The observed phenomenon can be, however, important only partially. Mainly, granulation experiments are interpreted empirically.
Several granulation methods have been proposed for solving the above described objects which, however, are not satisfactory for the stability of the obtained granules, or reduce the magnesium contents because of the addition of auxiliary matter, or have unfavorable cost of granulation processes.
In accordance with the solution proposed in the German Pat. No. 1,183,058, crystallization water of the kieserite is thermally removed before the granulation. As a result of this, the material becomes more active and can be hydrated faster. The primary particles in the granules are connected with one another because of the formation of hydrate. Disregarding the fact that the calcination causes high energy costs, the moisturized granulating mass is bound too fast, whereby the product grain yield is low. The German Pat. No. 2,106,212 proposes to bring calcium sulfate-magnesium sulfate hydrate in moisturized form into a rotary drum, to roll the same in the drum and heat to the temperature of between 250.degree. and 250.degree. C., so that the granules are reinforced by sintering. Here, also, an extremely high energy consumption is needed. The proposal disclosed in the German Pat. No. 2,316,703 is favorable from an energy point of view. Here fresh artificial langbeinite is added to potassium salt-kieserite mixture. This method is successful for granulation of predetermined calcium sulfate-kieserite mixtures. However, from the quality point of view it is not transferable to all conditions of mixing. Moreover, fresh langbeinite is not cost-favorable from all standpoints.
The method disclosed in the German Pat. No. 2,748,152 uses ammonium sulfate as granulation auxiliary material for kieserite. The moisturized granulating product is rolled and then dried over the surface. The rigidification is performed by a hydration during extremely long maturing time, and provides wear resistant granules. The maturing process is, however, very difficult to control in practice.
During granulation of kieserite in accordance with the proposal of the German Pat. No. 3,148,404, significantly better results are obtained. This is especially true when natrium sulfate is used as granulating auxiliary material. What is disadvantageous in this method is that a high quantity of additives is required during the operation, for obtaining sufficiently wear-resistant granules. For producing the hydrate binding, also significant water quantities must remain in product which, naturally lowers the nutrient content.
An important presumption for the granulation of kieserite, it is a wide grain spectrum with a high fraction of fine product under 0.09 mm. Thereby, during formation of individual granules by rolling, a high packing density is obtained which substantially contributes to mechanical strength.
Such a granulating product with ideal primary grain distribution is moisturized with water, sufficient grain strength is obtained during rolling which can be reinforced to form wear resistant structures by maturing or drying. In unexpected manner, such a product is stable under storage conditions not longer than three weeks. The bursting strength of the kieserite granules dried to 2% residual moisture in accordance with this method reaches a maximum after five days storage with 20 N/grain and falls after three-four weeks to 7 N/grain, while the wear in the test increases from 4 to 38%.
Kieserite mixtures with increased addition of calcium sulfate behave similarly, when not so completely extreme. For example, a mixture of approximately 23 weight per parts of calcium sulfate and 77 weight parts of technical kieserite with optimum grain spectrum reaches after the granulation and drying, in five days a bursting strength of 28 N/grain, which after four weeks falls to 19 N/grain, wherein the wear increases to 15%. Other disadvantageous observations have been made with respect to mixtures containing the calcium sulfate. Since the initially optimally moisturized granulating mixture tends to warming and to premature drying, the size distribution of the granules obtained by rolling becomes very unfavorable. The back product circulation is strongly loaded both because of a high undergrain fraction, and because of gravel-like under grain. The cause for these conditions is fast hydration of finest kieserite particles and their addition with the calcium sulfate to make double salt.
During granulation of mixtures with calcium chloride and a kieserite fraction over 30%, additionally an increased sensitivity to air moisture takes place. This can be explained by the hydroscopic ability of the magnesium chloride produced during formation of sulfatic double salt.
It was found in a surprising manner that the substances from the class of sugar and their hydrofilic derivatives delay the undesirable processes in moisturized kieserite mixtures and suppress the structure loosening in granules during long storage. The mixtures do not dry during rolling prematurely and show during storage, in contrast to the granules with the above additions, no blooming.